The present invention relates to a semiconductor photodetector and, more particularly, to a semiconductor waveguide photodetector.
Recently, replacing the existing electric cables with optical communication cables as subscriber communication means is being examined. In doing this, the largest technical difficulty is to inexpensively provide small-sized, highly reliable subscriber transmitting/receiving apparatuses. Also, in conventionally proposed transmitting/receiving apparatuses, it is being attempted to integrally form a light-emitting device 501, a photodetector 502, and an optical waveguide 503 on a single substrate 504, as shown in FIG. 5, in addition to miniaturizing electronic circuits by using ICs and reducing the consumption power. To integrally form these devices on the substrate 504 on which the optical waveguide 503 having an optical branch circuit 505 is formed, it is important to form waveguide structures in both the light-emitting device 501 and the photodetector 502. Note that reference numeral 506 in FIG. 5 denotes a monitoring photodiode.
As a photodetector having this waveguide structure, a semiconductor waveguide photodetector shown in FIG. 6 is being studied since this photodetector is suited to a high-speed operation.
In this semiconductor waveguide photodetector, a 0.6-.mu.m thick n-type InGaAsP optical guide layer 602 having a band gap wavelength of 1.3 .mu.m is formed on a semi-insulating InP substrate 601. In a predetermined region on this optical guide layer 602, a 0.6-.mu.m thick n-type InGaAs photoabsorption layer 603 with a low carrier density is formed. A 0.6-.mu.m thick p-type InGaAsP optical guide layer 605 having a band gap wavelength of 1.3 .mu.m is formed on the photoabsorption layer 603. A 0.5-.mu.m thick p-type InP cladding layer 606 is formed on the optical guide layer 605.
An n-type ohmic electrode 607 is formed on a region where the photoabsorption layer 603 on the optical guide layer 602 is not formed. A p-type ohmic layer 608 is formed on the cladding layer 606 (K. Kato et al., "A. high-efficiency 50 GHz InGaAs multimode waveguide photodetector", IEEE Journal of Quantum Electronics Vol. 28, No. 12, p. 2728, 1992).
The principle of the operation of this semiconductor photodetector shown in FIG. 6 is as follows. Incident light with a wavelength of 1.55 .mu.m incident from the cleavage surface is guided in an optical waveguide constituted by the substrate 601, the optical guide layer 602, the photoabsorption layer 603, the optical guide layer 605, and the cladding layer 606. While being guided in the optical waveguide, the incident light is absorbed by the photoabsorption layer 603 and converted into electrons and holes (O/E conversion). These electrons and holes produced by the O/E conversion are made run to the n- and p-type semiconductor layers, respectively, by an electric field generated by a reverse bias voltage applied to the pn-junction, and are extracted out from the device as a signal current.
In providing a small-sized, high-reliability transmitting/receiving apparatus at a low cost as described previously, a light-emitting device has no problem. However, when a waveguide photodetector is used as the photodetector for a subscriber transmitting/receiving apparatus, the following problems arise.
First, in the subscriber transmitting/receiving apparatus, it is necessary to simultaneously realize miniaturization of electronic circuits and reduction of consumption power. To this end, integrated circuits driven with a lower voltage must be used, so the driving voltages of a light-emitting device and a photodetector are lowered accordingly. Also, the voltage to be applied to a photodetector must be 2 V or less which is used in low-consumption-power integrated circuits, and is preferably 1 V.
Even a photodetector like this is naturally required to have a high O/E conversion efficiency (to be simply referred to as an efficiency hereinafter) and a high operating speed.
The semiconductor waveguide photodetector as shown in FIG. 6 has an optical waveguide structure and is in this respect suited to the integration of a transmitting/receiving apparatus necessary to realize optical communication in the subscriber system.
On the other hand, this semiconductor waveguide photodetector has a very high operating speed reaching 50 GHz, so the application to a large-capacity, high-speed transmission system used between switching systems has been exclusively examined. Therefore, as will be described below, the response speed is decreased when the driving voltage is lowered, and this makes this photodetector unsuited to miniaturize electronic circuits in the transmitting/receiving apparatus and reduce the consumption power of the apparatus.
First, the thickness of the photoabsorption layer must be 2 .mu.m or more for the reasons explained below. In receiving an optical signal about 10 .mu.m in diameter supplied through an optical fiber by using a semiconductor waveguide photodetector with a finite length, the efficiency of O/E conversion can be increased as the thickness of the photoabsorption layer increases. Additionally, in a semiconductor waveguide photodetector, to decrease the capacitance and allow an input optical signal (diameter=about 10 .mu.m) from an optical fiber to be efficiently coupled with an optical guide, the optical waveguide including the photoabsorption layer is so processed as to have a mesa structure (or a ridge structure) having a width of about 30 .mu.m. Furthermore, to decrease the capacitance to a desired small value, the length of the optical waveguide including the photoabsorption layer must be further decreased.
For the reasons as above, when the photoabsorption layer is thinned, signal light can no longer be well absorbed by the photoabsorption layer. When the device length is 100 .mu.m, for example, the efficiency of O/E conversion depending upon the thickness of this photoabsorption layer is 90% for a 3-.mu.m thick photoabsorption layer and 75% for a 2-.mu.m thick photoabsorption layer. To use the photoabsorption layer as a photodetector, an efficiency of 70% or more is necessary. Therefore, the thickness of the photoabsorption layer must be 2 .mu.m or more.
On the other hand, in a semiconductor waveguide photodetector, the photoabsorption layer is generally made have as a low carrier density as possible, e.g., an n-type low carrier density of about 1.times.10.sup.15 cm.sup.-3, for the sake of crystal growth. This is to deplete the photoabsorption layer as much as possible with an applied bias voltage and allow carriers (electrons and holes) generated by O/E conversion to run at a high speed.
The present inventors, however, have found the following problem. As shown in FIG. 7, with a low bias voltage of about 1 V, for example, a photoabsorption layer 701 having a carrier density of about 1.times.10.sup.15 cm.sup.-3 is depleted only to about 1.5 .mu.m. Consequently, in a device in which the thickness of the photoabsorption layer 701 is 3 .mu.m, an n-type layer 702 about 1.5 .mu.m thick remains in the photoabsorption layer 701 without being depleted. A hole 704 generated by O/E conversion slowly moves in this n-type layer 702 due to carrier diffusion and then enters a depleted region 703. Accordingly, in this state, it is impossible to respond to a high-speed signal of 10 MHz or higher.
In summary, in the conventional semiconductor waveguide photodetectors, the thickness of the photoabsorption layer must be increased to, e.g., 3 .mu.m in order to raise the efficiency of O/E conversion. However, if the thickness of the photoabsorption layer is thus increased, it becomes impossible to respond to high-speed optical signals. That is, when the conventional semiconductor photodetector as described above is used as a photodetector of a subscriber transmitting/receiving apparatus, it is difficult to simultaneously accomplish a high efficiency and a high operating speed.